Tvheadend

Hi,
I have a FritzBox with DVB-C and already used it with tvheadend in the past so the Box itself is working.
Now I've changed the network setup that behind the Fritzbox a new Router (TPLink TL-R605) is located with its own NAT and different subnet (and of course own DHCP and DNS). I can open ports for getting calls inside my new network (443 is opened and works). Also calls from the internal network to the Fritzbox (ping, port 80, port 49000,...) are working (and also to the internet).
My tvheadend server is inside my new net (running on Ubuntu). I've added as ARGS the satip_xml with the url of the satipdesc.xml on the Fritzbox. With that I see the 4 tuners in the UI of tvheadend.
specinv is set to not use. If I now add Muxes and let then scan I get the following:

2021-02-05 07:12:02.632 subscription: 0095: "scan" subscribing to mux "426MHz", weight: 5, adapter: "SAT>IP DVB-C Tuner #1 (192.168.178.1)", network: "fff", service: "Raw PID Subscription" 
2021-02-05 07:12:02.831 satip: SAT>IP DVB-C Tuner #1 (192.168.178.1) - RTSP cmd error 7 (Unknown error -7) [8-408]
2021-02-05 07:12:07.635 mpegts: 426MHz in fff - scan no data, failed
2021-02-05 07:12:07.635 subscription: 0095: "scan" unsubscribing

My fritzbox is on 192.168.178.1
the TL-R605 has the internal network 192.168.0.1/24 and has on the Fritzbox the IP 192.168.0.101
Tvheadend is running on 192.168.0.131
satip_xml is on http://192.168.178.1:49000/satipdesc.xml (and delivers the correct stuff)

If I use the m3u from the fritzbox I can watch TV insided the 192.168.0.1 network using VLC (so links like rtsp://192.168.178.1:554/?avm=1&freq=330&bw=8&msys=dvbc&mtype=256qam&sr=6900&specinv=1&pids=0,16,17,18,20,5100,5101,1170,1176,2171,2172,5102,5103,5104,5105 are working).

What do I have to do to get TVH running again? (If possible I don't want to add a Raspi in the 192.168.178.1 network)

Thanks and Kind Regards
Marco